1. Field of the Invention
The present invention generally relates to an image forming apparatus including a fixer.
2. Description of the Background Art
In various areas, an image forming apparatus, such as a copier, a facsimile machine, a printer, etc., that employs an electronographic method or an electrostatic recording method has been used. Such an image forming apparatus includes an image carrier on which an electrostatic latent image is formed, a developing device to develop the electrostatic latent image with a developer, a transferer to transfer the image onto a sheet of recording medium, and a fixer to fix the image on the sheet.
As an example, the fixer includes a pair of rollers facing each other, and one of these rollers is used as a heating roller and the other is used as a pressing roller to press the recording medium against the heating roller. In such a configuration, the unfixed toner image is fixed on the recording medium with heat from the heating roller and pressure from the pressing roller while the recording medium passes through a nip formed between these rollers.
Recently, a proportion of color images to total images output from such image forming apparatuses has been increasing, and with it increasing demand for higher quality images. One measure used to evaluate color image quality is gloss level of a toner portion. While images of lower gloss levels, for example, 15% or less, are generally preferred as business documents, images of higher gloss level, for example, 20% or greater, are generally preferred as printed materials including catalogues and brochures. Therefore, in some cases, it is necessary to use different types of image forming apparatuses depending on the purpose of use of output images.
Generally, to produce images of higher gloss levels, a fixing temperature and a fixing pressure are increased. However, in such a case, moisture in the recording medium is likely to vaporize and swell, causing spots in a toner image, which is a failure called blistering.
To prevent such blistering, fixers that perform preheating have been proposed. For example, in one known method, a fixer preliminarily heats the recording medium to 60 degrees Centigrade or higher when a coated paper discriminating means determines that the recording medium is a coated paper. In another known method, an upstream first fixer fixes a unfixed toner image on a first side of a recording medium and a downstream second fixer fixes a unfixed toner image on a second side of the recording medium. A heating temperature of the first fixer is lower than that of the second fixer, and the unfixed toner images on both sides of the recording medium are fixed in one fixing process. Further, in another known method, a plurality of fixing nips is formed with respect to a fixing belt, so that fluctuation in a belt transportation speed is prevented to achieve a uniform gloss level.
It is common to produce the toner for an electronograph through a kneading and pulverization method in which a thermoplastic resin is dissolved and kneaded together with a colorant and, if required, a releasing agent such as wax and/or a charge controller. This mixture is then pulverized and sorted. To surfaces of the toner particles thus obtained, inorganic or organic fine particles are added, as required, to improve fluidity and/or facilitate cleaning.
A toner produced through a known kneading and pulverization method generally has an indefinite form, a relatively broad particle distribution, relatively low fluidity, and relatively low transfer properties. The toner requires a relatively large amount of energy to be fixed, and a charge amount is rather uneven among the toner particles, and thus its charge stability is relatively low. Further, the quality of the images produced with such toner is inadequate.
As another method to produce toner, polymerization methods have been proposed. The polymerization methods do not include kneading and pulverization processes, and thus energy, production time, and cost can be reduced. Further, such a polymerized toner produced through polymerization has a particle distribution narrower than that of the toner produced through the pulverization method. Moreover, the fluidity of the polymerized toner can be enhanced significantly because capsulation of wax is relatively easy, and thus roundish toner particles can be produced relatively easily.
However, properties of such a polymerized toner are inadequate even though the polymerized toner particles are generally rounder than the pulverized toner particles because surface tension acts in the polymerization process. Further, controlling the ultimate shape of toner particles (deformation) is difficult in the polymerization method. As for the charge stability and the transfer properties, the polymerization methods are advantageous.
Among the polymerization methods, although a suspension polymerization method is widely used, in this method, a monomer for a binder (binder resin) is limited to styrene monomer, acrylic monomer, etc., which are harmful to humans. As a result, the toner produced through the suspension polymerization method includes such toxic components and is thus not environmentally friendly.
Further, because wax is included inside the toner particles in the case of the suspension polymerized method, the wax is less likely to appear on the surfaces of the toner particles when the toner is used, compared with the pulverized toner in which wax exist on the surfaces of the particles. Therefore, although the toner is less likely to adhere to the photoreceptor, the suspension polymerized toner has a lower fixing efficiency, thus consuming a greater amount of power for fixing than the pulverized toner does.
Further, in the case of the polymerized toner, when wax is increased in amount and/or dispersion particle diameter so as to enhance fixing, transparency of color image is impaired, and thus such a toner is not suitable for OHP films used in presentations.
Another method to produce a polymerized toner is an emulsion polymerization method that can deform particles to a certain level. In the emulsion polymerization method, the monomer to be used is also limited to styrene monomer, and removing an unreacted monomer, an emulsifier, and a dispersant from toner particles completely is difficult, and thus the toner is not environmentally friendly.
Yet another method to produce a polymerized toner is a dissolution suspension method that can use a polyester resin that is fixable at a lower temperature. However, because a component having a relatively large molecular weight is used in a process to dissolve or disperse such a resin and a colorant, viscosity of the dissolution or dispersion liquid is increased, and productivity is decreased.
Further, in the dissolution suspension method, toner particles are spherical and surfaces thereof are uneven so that the toner particles can be better cleaned. However, the shape of such toner particles is irregular, and charge stability thereof is relatively low. Moreover, durability and a release are relatively poor, and thus the quality thereof is inadequate.
A dry toner that has a practical sphericity within a range of from 0.90 to 1.00 has been proposed to improve fluidity, fixing at a low temperature (low temperature fixing properties), and resistance to hot offset. Hot offset is a phenomenon that occurs when toner is overheated, reducing cohesion among toner particles below that to a fixing roller or to paper. In such a condition, a toner layer separates, and toner in a resultant image will be partly absent.
Such a proposed toner uses a toner binder manufactured through an extending reaction of a urethane-modified polyester. Further, another dry toner has been proposed that has higher powder fluidity and transfer properties when the toner has a smaller particle diameter. This toner also excels in thermal and storage stability, the low temperature fixing properties, and resistance to hot offset. Methods to manufacture these toners include a molecular weight increasing process in which a polyester prepolymer containing an isocyanate group and amine perform polyaddition reaction in an aqueous medium.
However, in the polymerized toner obtained through the method described above, the colorant disperses unevenly, and transparency and saturation of images produced with such a toner is lower. The images are rather dark, particularly when images are formed on OHP films using such a toner,